For example, a related art wireless communications system currently disclosed by following nonpatent reference 1 is comprised of the following components.
(1) A Data Transmission Device
                an error correction encoding unit for carrying out error correction encoding of data which is a transmission object to create a codeword        a modulating unit for modulating the codeword created by the error correction encoding unit by using QPSK or the like.        an S/P conversion unit for carrying out serial parallel conversion of the codeword modulated by the modulating unit, and for dividing the codeword into codewords.        an inverse discrete Fourier transform unit for carrying out IDFT (inverse discrete Fourier transform) of the codewords obtained by the S/P conversion unit in the case of OFDM modulation.        an MIMO processing unit for simultaneously transmitting the codewords on which the IDFT is performed by the inverse discrete Fourier transform unit by using a plurality of transmitting antennas (the MIMO processing unit includes up conversion from a baseband to a high frequency band).(2) A Data Reception Device        an MIMO processing unit for receiving the plurality of codewords simultaneously transmitted from the data transmission device by using a plurality of receiving antennas (the MIMO processing unit includes down conversion from a high frequency band to a baseband).        a discrete Fourier transform unit for carrying out DFT (discrete Fourier transform) of the codewords received by the MIMO processing unit in the case of OFDM modulation.        a P/S conversion unit for carrying out parallel serial conversion of the codewords on which the DFT is performed by the discrete Fourier transform unit, and for unifying the plurality of codewords into a codeword.        a demodulating unit for demodulating the codeword into which the plurality of codewords are unified by the P/S conversion unit.        an error correction decoding unit for carrying out error correction decoding of the codeword demodulated by the demodulating unit to reproduce the data.        
In the case in which the MIMO processing unit of the data transmission device transmits a plurality of codewords simultaneously by using the plurality of antennas, an improvement in the transfer rate of the data, etc. can be provided. However, there is a case in which interference occurs among the plurality of communication channels.
Hereafter, a case in which a plurality of codewords are transmitted by way of three communication channels and interference occurs among the three communication channels will be explained.
First, a communication channel response matrix H is assumed as follows.
                    H        =                  [                                                                      h                                      1                    ,                    1                                                                                                h                                      1                    ,                    2                                                                                                h                                      1                    ,                    3                                                                                                                        h                                      2                    ,                    1                                                                                                h                                      2                    ,                    2                                                                                                h                                      2                    ,                    3                                                                                                                        h                                      3                    ,                    1                                                                                                h                                      3                    ,                    2                                                                                                h                                      3                    ,                    3                                                                                ]                                    [                  Equation          ⁢                                          ⁢          1                ]            where hi,j is a communication channel response from j-th transmitting antenna to i-th receiving antenna.
Furthermore, the symbols of the codewords transmitted from the MIMO processing unit of the data transmission device (symbols which are obtained by modulating an error correcting code expressed by two elements) are expressed as S. For the sake of simplicity, it is assumed that no OFDM modulation is carried out in this example.
                    S        =                  [                                                                      S                  1                                                                                                      S                  2                                                                                                      S                  3                                                              ]                                    [                  Equation          ⁢                                          ⁢          2                ]            
When the MIMO processing unit of the data transmission device transmits the symbol S of the codewords by using the three transmitting antennas, the MIMO processing unit of the data reception device receives a symbol of codewords by using the three receiving antennas. When the received signal is expressed as r, the received signal r is shown as follows.
                              r          =                                    HS              +              n                        =                                          [                                                                                                                                                          h                                                          1                              ,                              1                                                                                ⁢                                                      S                            1                                                                          +                                                                              h                                                          1                              ,                              2                                                                                ⁢                                                      S                            2                                                                          +                                                                              h                                                          1                              ,                              3                                                                                ⁢                                                      S                            3                                                                                                                                                                                                                                                        h                                                          2                              ,                              1                                                                                ⁢                                                      S                            1                                                                          +                                                                              h                                                          2                              ,                              2                                                                                ⁢                                                      S                            2                                                                          +                                                                              h                                                          2                              ,                              3                                                                                ⁢                                                      S                            3                                                                                                                                                                                                                                                        h                                                          3                              ,                              1                                                                                ⁢                                                      S                            1                                                                          +                                                                              h                                                          3                              ,                              2                                                                                ⁢                                                      S                            2                                                                          +                                                                              h                                                          3                              ,                              3                                                                                ⁢                                                      S                            3                                                                                                                                              ]                            +                              [                                                                                                    n                        1                                                                                                                                                n                        2                                                                                                                                                n                        3                                                                                            ]                                                    ⁢                                  ⁢                              where            ⁢                                                  ⁢            n                    =                      [                                                                                n                    1                                                                                                                    n                    2                                                                                                                    n                    3                                                                        ]                                              [                  Equation          ⁢                                          ⁢          3                ]            is a sequence consisting of noise components of receiving antennas.
In this case, when demodulating S1, S2, and S3 which construct the symbol S of the codewords from the received signal received by the receiving antennas 1, 2, and 3, the receiving point of the receiving antenna 1 expressed on the complex plane is shown in FIG. 3(i) at the time of the first-time transmission of the symbol S (according to a simple decoding method, both interference and noise are assumed to be degradation components).
At this time, the received signal is shown as follows.(h1,1S1+h1,2S2+h1,3S3+n1)h1,1*=S1+h1,2h1,1*S2+h1,3h1,1*S3+h1,1*n1   [Equation 4]where h1,1h1,1*=1, and h1,2h1,1*S2+h1,3h1,1*S3+h1,1*n1 is all noise components at time of demodulation.
The data transmission device has a mechanism of implementing HARQ-CC (Hybrid Auto-Repeat reQuest-Chase Combining) as a mechanism of, when the data reception device fails to receive the symbol S, automatically transmitting the same symbol S again.
When the same symbol S is retransmitted thereto, the data reception device adds the symbol S to that received at the time of the first-time transmission to calculate the average, thereby reducing the energy of the noise on average by 3 dB to raise the success rates of the demodulation and error correction decoding.
However, as shown in FIG. 3(ii), because interference wave components including h1,2h1,1*S2+h1,3h1,1*S3 remain without being reduced, there are few improvements in the performance (according to the conventional method, only the noise becomes small because of HARQ-CC).
[Nonpatent reference 1] IEEE C802.16m-07/189